Chapter 5. 802.11ac Planning

If you don’t know where you’re going, you’ll end up someplace else.

—Yogi Berra

Although most of the discussion in this book has been about speed, the
real value of 802.11ac to the network administrator is that it increases the
capacity of a wireless network. Whether the network needs to serve more
clients with today’s level of throughput or today’s client load with higher
throughput, the solution is 802.11ac.

Several intersecting trends are driving the need for increased
capacity. Many new devices are built around the assumption that 802.11
coverage is ubiquitous and therefore do not have an alternative LAN
technology for accessing networks. Of these new devices, most of them are
battery-operated and portable, and do not even have the capability to
connect to wired Ethernet networks. As traffic shifts onto the wireless LAN,
it must support new demands for connectivity. Increased numbers of devices
is only the first part of a one-two punch being delivered by users. After
connecting so many devices to wireless LANs, users then change the type of
applications in use. With improved computing power and display technology,
the user experience is becoming significantly more media-heavy, with a
special emphasis on streaming multimedia and especially video support.
Combine an increase in the number of devices with increased demand for
capacity from each device, and you have a recipe for congestion unless
greater capacity is in the cards. As the improved performance of 802.11ac
becomes readily available in client devices, there will be user demand to
take advantage of that speed.

Adoption of 802.11ac will likely happen more quickly than that of its
predecessors. Improving speed is always welcome in networking, and many
networks are built with a three- to five-year time horizon of service. Part
of the planning process in building an 802.11ac network is to assess not
only the current load on your network, but also the expected growth in
demand for service to determine whether the increased density justifies
using the highest-performance technology available. A strong industry focus
on interoperability has made the transition to 802.11ac straightforward for
network administrators as well.

Getting Ready for 802.11ac

802.11ac is evolutionary as much as it is revolutionary. Many of the
design principles that have been used with previous technologies are still
applicable, with a few minor changes to take advantage of new protocol
features. The drivers to use 802.11ac are the same drivers that have
justified every other network upgrade you have ever done:

Peak speed and/or throughput

The most obvious driver for 802.11ac is the new higher
speeds. Some applications require as much speed as the network can
deliver, and these are obvious beneficiaries of the new
technology. Increased use of video is a major driver of 802.11ac
adoption, as is the increase in device density due to the
widespread use of tablets and wireless LAN–equipped smartphones.
Video is widely used throughout the spectrum of wireless LAN
users, whether it is large and detailed images for patient care,
instructional videos in the classroom, or wireless display
technologies in corporate conference rooms. Higher speeds also
enable additional point-to-point deployment scenarios and provide
the capacity necessary to serve 802.11n clients with mesh backhaul
connections.

Capacity

With so much raw capacity, especially with wider channels,
802.11ac provides a superior level of service. In addition to the
general efficiencies that the IEEE 802.11 working group builds
into new specifications, products often add clever features to
further extract capacity increases from the new physical layer.
One common method of doing so is to bias transmissions toward
frames that require shorter times to transmit. Even though
802.11ac can transmit large numbers of bits, the extremely high
data rates mean that even very large amounts of data are
transmitted faster than small packets were in 802.11b.

Latency

Some applications benefit primarily from lower latency,
especially real-time streaming applications such as voice,
videoconferencing, or even video chat. Improving latency can be
done by building a more efficient network, but often the best way
to improve latency is to reduce the load on the network. 802.11
measures load by airtime utilization, so moving to faster physical
layer standards improves latency by reducing the airtime load.
Multi-user MIMO also has the potential to decrease network load by
enabling parallel transmissions. Reducing latency means that even
a few 802.11ac devices may benefit the entire network by
decreasing airtime demand.

As part of the IEEE project authorization process, a task group in
the formation process needs to discuss compatibility with previous
technology standards. Early adopters of wireless LANs made significant
investments in the technology, and the IEEE process is designed to protect
that investment. Backward compatibility with prior 802.11 standards was a
key consideration in the 802.11ac standardization process, and there was
extensive work done in the protocol to ensure that 802.11ac would work
with the many existing wireless LAN devices. In addition to physical-layer
compatibility, 802.11ac has extensive MAC-layer compatibility, which
enables newer 802.11ac devices to perform at their best even when
surrounded by older devices. In fact, these functions were designed to
enable a little bit of 802.11ac to speed up any network.

802.11ac was designed from the beginning to be compatible with
prior standards (802.11n, 802.11a/g, and 802.11b). Don’t let
compatibility worries slow you down—adding 802.11ac speeds up any
network, even if it has only a few 802.11ac client devices.

Even though 802.11ac is the future physical layer in wireless LANs,
it will not be the only physical layer. APs that are sold as “802.11ac
APs” will have one 5 GHz radio running 802.11ac, and they will also have a
second 2.4 GHz radio running 802.11n. Even as 802.11ac becomes
established, the 2.4 GHz band will continue to depend on the same 802.11n
technology that has been used for the past several years.

Catching the 802.11ac Technology Wave

Early in the development of wireless LAN technology, a new PHY was
brought to market all at once. With 802.11n, however, the standards
started to become much more complex, and different levels of capability
came to the market in distinct “waves” or “phases.” Once the basic
technical details are worked out, it can often be much easier to write a
standard than to build a product. For example, the work required to add
four-spatial-stream support into the 802.11n standard was relatively
minimal after the basic ground rules were complete, but as of the 2013
publication date of this book, four-stream 802.11n devices have yet to
be brought to market because of the engineering challenges involved in
building the powerful DSP required to perform the spatial mapping while
staying within the 15-watt 802.3af power limit.

802.11n came to the market in waves due to the overall complexity
of the standard. 802.11ac will follow this well-worn path, with a rough
estimate of the contents of the first two waves in Table 5-1. The first generation of 802.11ac
delivers another jump in channel bandwidth, along with a new modulation.
Taken together, these two features are enough to nearly double the speed
of a typical three-stream client device. The second wave of 802.11ac
will add even wider channels, four-stream support, and beamforming.
Although there is a temptation to focus on the headline rates only,
beamforming has the potential to deliver significant gains in network
capacity by improving the data rates at which most clients transmit. Not
all transmissions occur at the fastest rate, so the beamforming boost
can be substantial if it increases the data rates used by
clients.

Table 5-1. 802.11ac technology waves

Wave 1

Wave 2

Standard basis

802.11ac, draft 2.0

802.11ac, final version

Timeframe

Mid-2013

2014

Channel width

20, 40, and 80 MHz

Potential to add 160 MHz channels

Modulation support

Up to 256-QAM

Same as wave 1

Lowest 11ac speed

173 Mbps (20 MHz, 2-stream, 256-QAM)

Same as wave 1

Typical 11ac speed

867 Mbps (80 MHz, 3-stream, 256-QAM)

1.7 Gbps (160 MHz, 3-stream, 256-QAM)

Maximum 11ac speed

1.3 Gbps (80 MHz, 3-stream, 256-QAM)

3.5 Gbps (160 MHz, 4-stream, 256-QAM)

Beamforming

Yes (depending on underlying chipset)

Yes, possibly MU-MIMO

First wave 802.11ac versus second wave 802.11ac

A key decision in planning for 802.11ac is when to jump in and
deploy widely. Unlike previous physical layers in 802.11, the first
wave of 802.11ac does not offer a clear-cut compelling advantage for
every user. First-wave 802.11ac products are now available, and derive
their additional speed from two main protocol features. Getting the
most out of the first wave of 802.11ac will require an environment
that can use one or both of these features:

256-QAM

The two top data rates in 802.11ac add 33% to the speed
over 802.11n, but they require significantly higher
signal-to-noise ratios. As a practical matter, such high SNRs
require clean radio spectrum and short AP-to-client
distances.

80 MHz channels

Clean spectrum is required to allocate contiguous 80 MHz
blocks, and even with Dynamic Frequency Selection (DFS) support,
there will only be five available 80 MHz channels until the new
spectrum discussed in the sidebar “Proposed Additional Spectrum for 802.11ac in the United
States” becomes available. Five
channels is enough to plan a network, but it will not be as easy
as it was with the multitude of channels that were available in
802.11n.

In some environments, it is possible that neither of these
features will provide a compelling reason for a widespread 802.11ac
deployment. In that case, it still offers the highest available
capacity and best support for high-density areas within your network.
Table 5-2 compares the
performance of the first two waves of 802.11ac.

Table 5-2. Performance comparison of 802.11ac waves

Protocol feature

First-wave gain over 802.11n

Second-wave gain over 802.11n

256-QAM data rates

1.33x

1.33x

80 MHz channels

2.1x

Same as first wave

160 MHz channels

Not available

4.3x

Up to eight spatial streams

No gain—first wave is 3 SS

1.33x—second wave is 4 SS

Multi-user MIMO

Not available

~2x?

TOTAL

2.8x

~15x?

Client Device Mix

As much as network administrators would like to believe that
networks are their own reward, a network exists to get work done. The
number, types, and capabilities of devices attached to the network are
an important part of the planning process. One set of data for input
into the planning process would be information on the existing devices
attached to your wireless LAN today, and your existing wireless network
management system should report the client mix in a variety of ways.
However, in building a network, it is important to look ahead over the
life of the network. In 2013, for example, only a few client devices
will be 802.11ac-capable, but within a year 802.11ac will be widely
available in client devices. Previous physical layers for wireless LANs
have followed similar adoption trajectories. At first, the new
technology is used in high-density and high-capacity areas; as those
areas take hold, they support enough of a volume increase to drive down
the cost of the new technology for everybody.

Table 5-3 shows the
evolution of client capabilities as they move from 802.11n to the first
wave of 802.11ac technology. Naturally, there will be departures from
the table, but the general rule is that high-end laptops will use the
fastest connectivity available while small battery-powered devices will
use power-efficient single-stream interfaces. Low-end laptops fall
somewhere in between and will typically settle for a less expensive
wireless interface that has middle-of-the road capabilities. High-end
tablets may also opt for two-stream interfaces.

Table 5-3. Effect of 802.11ac on client capabilities

Type of device

Radio type (in 2013 & earlier)

Channel width (2013 & earlier)

Data rate (2013 & earlier)

Radio type (2014)

Channel width (2014)

Data rate (2014)

Dual-band smartphone

802.11n, 1-stream

20 MHz

72 Mbps

802.11ac, 1-stream

20/40/80 MHz

Up to 433 Mbps

VoIP handset

802.11a/b/g or 1-stream 802.11n

20 MHz

54 Mbps

802.11a/b/g or 1-stream 802.11n/ac

20 MHz

Up to 87 Mbps

Tablet

802.11n, 1-stream

20/40 MHz

72 or 150 Mbps

802.11ac, 1-stream

20/40/80 MHz

Up to 433 Mbps

Netbook/low-end laptop

802.11n, 2-stream

40 MHz

Up to 300 Mbps

802.11ac, 2-stream

80 MHz

867 Mbps

High-end laptop

802.11n, 3-stream

40 MHz

Up to 450 Mbps

802.11ac, 3-stream

80 MHz

1.3 Gbps

Although 802.11ac is often dismissed as too power-hungry for
mobile devices, single-stream 802.11 MIMO devices do not require
significantly more power than their SISO predecessors. The main consumer
of power in a MIMO device is the power-hungry digital signal processor
that performs spatial mapping. By using only a single spatial stream, a
portable device can reap significant benefits from 802.11ac’s increased
speed and wider channels without paying a significant power-consumption
penalty. Although there will be an increase in power requirements to use
wider channels, the trade-off is that transmissions go so much faster
that the analog section is on for much less time. With a net battery
life benefit, 802.11ac will be adopted widely in portable devices. In
fact, 2013 saw the first introduction of an 802.11ac-capable
smartphone.

Information on your device mix can be gathered from several
sources. Naturally, knowledge of what has been purchased is an important
source of information, but with the trend away from supporting
exclusively corporate-owned devices, there is a need to gather
information on all of the devices using the network. One constraint on
the adoption of 802.11ac is that it is supported only in the 5 GHz band,
and a significant number of devices must be ready to move to the 5 GHz
band to see strong benefits from 802.11ac.

Because 802.11ac is only available in the 5 GHz band, the
benefits available depend on the number of 5 GHz–capable devices on
the network.

One welcome development of 802.11ac is that it is driving
increased use of the 5 GHz band. Many high-end client devices have begun
to support 5 GHz operation with dual-band 802.11n interfaces, and these
devices reward their users with improved connectivity. Use of the 5 GHz
band has been restricted to high-end devices, in large part because it
is still possible to be an “802.11n” device while supporting only one
band. In order to label a device as “802.11ac,” it will be necessary for
that device to support the 5 GHz band, even though it is almost certain
that a device labeled as “802.11ac” will also support 802.11n operation
in the 2.4 GHz band.

Supporting client devices in the 5 GHz band requires a somewhat
denser network deployment. If you have designed your network around the
needs of coverage for the 2.4 GHz band, successfully moving to 802.11ac
will require more APs.

Single-Stream Devices in 802.11ac

802.11ac offers significant benefits to single-stream devices.
By extending the channel width up to 80 MHz, it makes substantial
speed increases possible for single-stream devices, especially when
there is sufficient uncongested spectrum available to transmit with
wide channels. Looking forward to the next wave, multi-user MIMO has
the potential to add significant performance to networks as well by
transmitting to multiple single-stream clients at the same time.
Unlike with 802.11n, there is no need to discount the gains of
802.11ac simply because of the presence of a significant number of
single-stream client devices on the network. Even the first wave of
802.11ac APs will offer benefits to pre-11ac single-stream devices. As
new generations of radio chips are produced, the performance will continue
to improve, especially when MU-MIMO is available so multiple single-stream
802.11ac devices can receive transmissions simultaneously.

Application Planning

To be successful, the network must support the key applications
that are in use. Many access points now offer some form of application
visibility to augment your suppositions about the applications commonly
used on the network, and can report on the throughput used by common
applications. As an alternative to running application reporting on your
network, Table 5-4 has a list of
some of the most common applications that network administrators need to
consider, along with the Wi-Fi
Multimedia (WMM) access category that each application is typically
assigned. WMM allows administrators to place traffic into four
categories, with the higher-level categories receiving preferential
access to the medium. In declining order of priority, traffic can be
placed into queues for voice, video, best effort, or background
traffic.

Table 5-4. Application throughput needs

Application

Recommended bit rate (Mbps, unless
noted)

WMM access category

VoIP – voice transport

27 – 93 kbps (codec dependent)

Voice

VoIP – signaling (typically SIP)

5 kbps

Best effort

Remote display

150 kbps (without video), 1.8 Mbps (with video)

Video

Web conferencing

384 kbps – 1 Mbps

Video

FaceTime

0.9

Video

AppleTV video streaming

2.5 – 8

Video

High-definition video (compressed)

2 – 5

Video

High-definition video (uncompressed)

20

Video

High-definition video (uncompressed HDMI)

3.3 Gbps

Video

Standard-definition video

1 – 1.5

Video

Email/web browsing

0.5 – 1.0

Best effort

File sharing

5

Best effort

YouTube

0.9

Best effort

Network backup

Available capacity

Background

The applications in Table 5-4 are
all based on unicast data. In many cases, 802.11 access points will
convert multicast frames to unicast frames, and the same estimation
technique can be used for multicast applications.

Application throughput requirements can be used to create a rough
guide for the capacity requirements of an access point. None of the
applications in Table 5-4 is the
classic “killer app” that absolutely requires 802.11ac, but the
increased use of video distribution highlights the more limited capacity
of 802.11n. The easiest way to use application throughput requirements
to estimate capacity requirements is to divide the total capacity of a
device by the application's bit rate; this will give you a rough
estimate of the capacity needed. Although an 802.11ac AP may be capable
of nearly 1 Gbps of throughput, a single-stream tablet will be unable to
use all of that capacity. For example, if a single-stream device is
capable of 25 Mbps of TCP throughput, it will require approximately 4%
of the available airtime of an access point to do standard emailing/web
browsing (1 Mbps for the application divided by the 25 Mbps capacity). A
dual-radio AP could support approximately 50 such devices running the
application. For 802.11ac, there may be different capabilities in the
2.4 GHz 802.11n radio and the 5 GHz 802.11ac radio, provided the target
devices can use at least some of the advanced 802.11ac protocol
features.

Admission control

If a significant fraction of the anticipated traffic is in the
high-priority voice and video queues, part of your equipment
evaluation should be about whether admission
control is a valuable addition to your network. When
admission control is enabled, client devices must request access to
high-priority queues. For example, before placing a voice call, a
client must send a request to the AP to reserve capacity for a VoIP
data stream. The AP can then determine whether there is sufficient
airtime available to accept the device, and either reserve the
capacity or reject the request to connect due to insufficient airtime.
Admission control is available using a feature of the 802.11 protocol
called the Traffic Specification (TSPEC), and
products supporting this capability can be certified for
Wi-Fi Multimedia Admission Control (WMM-AC) by
the Wi-Fi Alliance interoperability certification program.

Physical Network Connections

As part of building an 802.11ac wireless edge, it is necessary to
connect APs to the edge of the existing network. This involves two main
tasks: physically connecting the AP to the edge of the network to
provide data transport services to it, and providing sufficient power to
start up the AP.

Backbone connectivity

Physical connections of 802.11ac devices to the backbone are a
snap. APs work as bridges and connect to existing Ethernet backbones,
so any existing Ethernet can readily be extended with 802.11. Even
basic two-stream 802.11ac devices can easily push more than 100 Mbps,
so a gigabit backbone is a practical requirement for an 802.11ac
access layer. Although some products will support bonding of multiple
links, Fast Ethernet just isn’t fast enough to support 802.11ac.
Upgrade your network edge to gigabit speed before installing
802.11ac.

Although 802.11ac is often described as “gigabit wireless,” a
gigabit Ethernet connection to the AP is sufficient for the first wave
of 802.11ac products. 802.11 speeds are based on the data rate used to
transmit the MAC frame, and do not include the effects of protocol
overhead such as interframe spacing and the need to transmit PHY
headers. Unlike Ethernet, 802.11 is a half-duplex medium. When an
Ethernet link is described as 1 Gbps, it is capable of operating at 1
Gbps in both directions, whereas its 802.11 equivalent is capable of
operating at 1 Gbps in both directions combined. Depending on network
traffic, the wireless LAN may have more upstream or more downstream
traffic, but the speed of the wireless LAN is the sum of the upstream
and downstream directions. To make speed even more
deployment-dependent, the top data rates in 802.11 are generally
available only to clients with high signal-to-noise ratios, and there
is a natural distribution of access speed because as devices increase
in distance from the access point the speed decreases. For 802.11ac,
speeds in excess of a gigabit require high-SNR links in order to use
the 256-QAM modulation, and the natural spatial distribution of
clients ensures that many clients will be operating at mid-range
speeds.

In most networks, the protocol overhead plus the spatial
distribution of client devices away from the AP will typically lead to
a maximum practical throughput of about two-thirds of the headline
rate. Apply that rule to a first-wave 802.11ac AP with a 1.3 Gbps
radio in the 5 GHz band and a 450 Mbps 802.11n radio for the 2.4 GHz
band, and the maximum practical throughput is slightly in excess of 1
Gbps. Even with atypical mixes of upstream and downstream traffic,
fitting that into a single full-duplex Ethernet link is
comfortable.

For the first wave of 802.11ac, make sure the Ethernet edge
is gigabit, but don’t worry about upgrading to 10-gigabit Ethernet
access ports.

Capacity analysis for the connection of the access layer is an
important component of ensuring sufficient backbone capacity. Although
gigabit connections suffice for connecting access points in the first
wave, the access layer switches themselves should have 10-gigabit
uplink capacity to the core of the network to accommodate multiple
802.11ac APs. As the capacity of 802.11ac grows in successive waves,
10-gigabit uplink capacity will become even more important.

As part of planning a first-wave 802.11ac deployment, you will
want to look ahead to the second wave in 2014. Cable infrastructure
needs to support a wireless LAN for much longer than the lifetime of
any particular generation of access points. With the second wave of
802.11ac, the speed will rise to 1.7 Gbps in 80 MHz channels and may
be as high as 3.5 Gbps if 160 MHz channel support is introduced. With
those speeds, a single gigabit link may no longer be
sufficient.

Several options exist for supporting the increased capacity of
the wireless LAN in the second wave. One is to handwave and say that
gigabit connections are sufficient, much like some network
administrators used Fast Ethernet to support early 802.11n APs. In
many cases, the actual connection rates will be low enough that this
might be viable, especially in coverage-oriented deployments.

If cable installation is required for your first-wave 802.11ac
deployment, it is possible to lay the foundation for the second wave
and beyond by installing two Ethernet cables to each AP location to
support bonded connections. Be sure to use high-quality cables such as
Category 6 or 7. The practical throughput of a 3.5 Gbps 802.11ac radio
plus a 600 Mbps 802.11n radio is probably around 2.5 Gbps total at
peak, but if the 160 MHz channel support is removed, the practical
throughput is probably more like 1.5 Gbps, a speed well within reach
of a dual bonded gigabit Ethernet connection. If you have an existing
cable plant, it is likely to be expensive to return to the cable plant
to add a second Ethernet link, but if the cable installation is new
with the 802.11ac deployment, it's a good idea to install two cables
just to be safe. The major cost of installing cable is labor, and the
decision to install two cables will not add significantly to the
cost.

For new cable plants to support 802.11ac, install two Ethernet
cables. Bonded 1-gigabit Ethernet connections are future-proof and
will support the second wave of 802.11ac.

Depending on your deployment scenario, there are additional
reasons to consider two ports. As wireless LANs continue their march
toward being the only access method for many devices, providing a
highly redundant service becomes even more important. Dual-homed
access points can draw power and connect to the network core through
redundant paths, which may be attractive for certain types of
deployments. For example, a financial firm that conducts trading
operations or a health care organization supporting patient care and
monitoring over a Wi-Fi network will want to carefully guard against
even brief outages.

As 802.11ac continues to evolve, even higher speeds may be
required. At the time this book went to press, 10-gigabit connections
were only available over fiber cables. Fiber does not support power
transmission, and thus is unlikely to be offered as an AP connection
technology. There are efforts underway to supply power over 10 gigabit
copper connections, but at the time this book was written, even
10-gigabit switches with copper connections were not very
common.

Power requirements

The electrical power requirements of 802.11ac will be higher
than for previous 802.11 standards. Although 802.11ac radio chips are
more efficient than prior chips, they are doing significantly more
work. Additional spatial streams and wider channels require more
sophisticated signal processing, so gains in power efficiency are
outweighed by the new protocol capabilities. With higher data rates,
frames are shorter and there is a significantly higher frame rate. All
this adds up to higher resource requirements at the AP: more power for
new radios, more buffer memory for frame operations, and
higher-powered CPUs to do more to each packet at higher frame rates.
As a result, 802.11ac APs are unable to work within the 13-watt budget
of 802.3af.[39]

802.11ac APs will not offer full functionality with 802.3af,
so part of the planning process should be to identify how to provide
the required power to new APs.

Power options for 802.11ac are basically unchanged from previous
generation of wireless LAN access points. The easiest way to provide
additional power to run 802.11ac APs is to provide power using 802.3at
(sometimes called “PoE plus”), a newer power standard that provides up
to 25.5 watts at the end of a full-length Ethernet cable. 802.3at
power is provided by many newer edge switches and can be added onto
existing networks by using mid-span power injectors.

Alternatively, APs can be powered by DC power adapters if there
are outlets readily available at the installation locations. If power
outlets are unavailable, it will probably be quite expensive to add
them to the best locations for AP installation, which are typically in
the ceiling. Some products have the ability to draw power
simultaneously from multiple power over Ethernet (PoE) connections,
which enables these products to add two 13-watt 802.3af sources
together for higher power draw. In most cases, the cost of running a
second cable to existing AP mounting locations is prohibitive compared
with that of purchasing mid-span injectors.

Security

802.11ac does not make fundamental changes to the 802.11 security
architecture, nor does it introduce new features that require
significant changes in your existing network security systems. Any
network security devices in place for an existing wireless LAN will
continue to work after an upgrade to 802.11ac unless they need to access
the wireless medium directly. The biggest change to network security in
802.11ac might be based on the equipment you choose to use for
802.11ac—i.e., you might want to install equipment that offers new
per-user capabilities that your previous network equipment did
not.

Link-layer encryption

802.11ac does not support the use of anything other than
AES-based encryption (CCMP and GCMP) to protect data frames.[40] To take advantage of the fast data rates in 802.11ac,
you will have to retire any TKIP-based networks. Many 802.11ac devices
will continue to support TKIP for client operations, but when doing so
will limit transmission rates to pre-802.11ac data rates. To lift the
cap on network capacity, you will need to convert the network over to
a new encryption method.

Many 802.11ac devices will support TKIP, but will only do so
with older performance-limiting 802.11a/b/g rates.

One method of transitioning away from TKIP is to run parallel
networks on the same infrastructure by duplicating an existing TKIP
network on newer APs. By monitoring the usage of the TKIP network, it
is possible to determine when enough older devices have been retired
and the TKIP-compatible network may be decommissioned. As an
alternative to parallel networks, both encryption protocols can be run
simultaneously on the same SSID, which is sometimes called
mixed-mode operation. In a mixed-mode network,
the encryption method must be supported by all clients—in this case,
this means the lowest common security denominator of TKIP will be
used, which will limit performance, especially for applications that
make extensive use of broadcast and multicast traffic.

Fast roaming

Real-time applications such as voice and videoconferencing
require uninterrupted access to the radio medium, even when moving
between APs. Therefore, the ability to move connections rapidly
between APs is critical for real-time applications such as voice and
videoconferencing. When security must be included as part of the
handoff between APs, there are two major implementation paths.
Opportunistic Key Caching (OKC) moves the master key between APs and
is widely available in network equipment. The 802.11r specification
also provides a guaranteed fast transition capability and is the
foundation of the Wi-Fi Alliance’s Voice-Enterprise certification
program.

Management frame protection

In 2009, the 802.11 working group ratified 802.11w, a standard
for the protection of management frames. Unicast management frames are
protected with CCMP and encrypted to prevent eavesdropping, while
broadcast management frames are authenticated with the
Broadcast/Multicast Integrity Protocol (BIP). 802.11ac has no mandates
regarding management frame protection, but it is likely that the
initial 802.11ac products will be some of the first available products
with management frame protection. Therefore, you should consider
whether to use management frame protection on your network. Management
frame protection can be operated in one of two modes:

Management frame protection capable

In this mode, an AP will advertise that it can protect
management frames. If a client that supports management frame
protection attaches to the network, the AP will encrypt
management traffic to it.

Management frame protection required

In this mode, an AP advertises not only that it can
protect management frames, but also that clients must support
the capability to use the network. If a client is unable to
support management frame protection, it will not be allowed to
connect to the network.

Management frame protection is potentially a worthwhile
capability if you are using devices that make extensive use of
management frames, such as devices that support the Wi-Fi Alliance’s
Voice-Enterprise certification.

Authentication

802.11ac made no changes to the 802.1X authentication framework.
Any user authentication system that works with 802.11a/b/g/n networks
will also work with an 802.11ac network.[41] EAP-based authentication is designed to work on top of
many different physical layers, and therefore it does not require any
changes when moving to 802.11ac. Connections between the wireless
network and the user account system should not need to be
redesigned.

Additional Planning Considerations

Wireless networks do not have many vendor-independent management
tools and protocols. An important part of planning a network and
evaluating equipment is to assess the vendor management tools that are
typically tightly integrated with the APs. Management tools typically
perform both configuration management and ongoing monitoring.

To develop a way of assessing products, it will help to devise
usage scenarios for what the network must support. Almost universally, a
wireless network needs to support employee access as well as guest
access. Commonly, employee access will be differentiated in some
fashion, such as by user role or device type. Contractors and
consultants may be given even more restricted access.

Guest management

Wireless networks are so useful that they often are key
infrastructure for additional services offered by the IT team. One of
the most notable examples is guest services, which may be composed of
guest registration, authentication, and billing, or some subset of the
three. Now that mobile devices almost universally use wireless LANs to
access the network, wireless LAN deployments are often used to provide
guest access to visitors. An adjunct to many wireless LAN deployments
is a guest management system that is used to manage accounts for
visitors.

Guest management systems have recently taken on a related role
as a differentiator between corporate-owned devices and employee-owned
devices. Enthusiasm for bring-your-own-device (BYOD) programs is based
on the productivity increases that flow from putting information quite
literally in the hands of users. Designing a technical architecture
for a BYOD program is a book topic in itself; one of the core
technical problems that must be solved is finding a way to distinguish
corporate-owned devices from employee- or visitor-owned devices so
that different policies can be applied to these sets of devices. In
addition to flexible security models and policies, a BYOD program may require building a network
that requires a significantly higher level of service due to increases
in device density.

Intrusion detection

Wireless intrusion detection systems were once considered a
standard part of the network administrator’s toolkit, due to the
relatively weak security mechanisms available to wireless LAN devices
in the era before 2003. The improved cryptographic capabilities
available for both data protection and management frame security have
mitigated known attacks, and most wireless LAN system vendors have
moved to integrate containment capabilities into their product lines
by controlling the wired network.

802.11ac Radio Planning

With planning complete, it’s time to pick out equipment to build the
network. Developing solid requirements, as outlined in the previous
section, is an important step in understanding what the network needs to
do. Many of those requirements can be translated into a tentative plan
that helps guide selection of hardware. Good project management practices
are somewhat iterative. Begin with a rough estimate of your network
requirements, and short-list vendors that can help meet your requirements.
Bring in demonstration equipment to prove out the basic design, and gather
information to refine your rough estimate. Above all, don’t be afraid to
put some load on your network as you are proving the concepts.

Available Radio Channels

802.11ac uses the 5 GHz spectrum exclusively, and at the time this
book went to press, it had 22 available 20 MHz channels for use. In
deployment practice, 14 of those channels require the use of Dynamic
Frequency Selection (DFS) to protect radar operations. At an 80 MHz
channel width, however, the number of available channels shrinks to just
five, and three of those five channels require DFS support. Although the
number of channels is reduced substantially, five channels is still
sufficient to provide channel separation in almost any area that will
see a wireless LAN deployment. Once the proposed spectrum expansion
described in Chapter 2 is finalized, four more 80
MHz channels will be added.

Coverage and Capacity Estimates

An important step in the planning process is to estimate the
number and type of APs that you will need to build your network. The AP
count can be estimated for a network that provides basic coverage, or it
can be estimated based on the capacity or transaction requirements
identified for the network. Both types of estimates are important,
especially for dense networks that have significant numbers of hot spots
with high user density. “Coverage-oriented” networks provide basic
connectivity for a low density of devices and can be built successfully
without advanced features. Increasingly, however, networks are being
built around capacity, and 802.11ac is the core technology that will
enable the next generation of “capacity-oriented” networks. If you are
not building a network around high capacity, you probably do not need
802.11ac. Table 5-5 is a basic
comparison of the two approaches to building a network.

Table 5-5. Network characteristics

Attribute

Low-density network

High-density network

Number of clients supported per AP

Low

High

Typical distance between APs

Higher

Lower

Floor area covered by an AP

5,000 square feet (500 square meters) or more

2,000–3,000 square feet (200–300 square meters)

802.11 physical layer type

802.11n

802.11ac

Typical signal strength and
signal-to-noise ratio in AP handoff area

–80 dBm (about 20 dB SNR)

–67 dBm or higher (about 33 dB SNR)

Radio design

Optimized for area of coverage

Optimized for throughput per unit of coverage
area

Target frequency band

2.4 GHz (sometimes 5 GHz)

5 GHz

Load balancing/band steering

Not needed due to common lack of dual-band client
devices

Required

Quality of service

Not needed

Required

Application mix

Light usage of best effort data

Voice and/or video are present

Even within a single network, both approaches may be used
depending on the area. When planning out a network, designers will need
to mix the two approaches to make it successful. Stairwells and hallways
are often areas where users need connectivity while in transit, but the
user density and application demands of the typical stairwell are quite
small compared to those of conference rooms, auditoriums, and office
space. In such sparsely used areas, it is acceptable to design for lower
capacity and a more moderate signal quality, perhaps even using less
expensive 802.11n access points.

Turning the raw data of network devices and applications into a
running network requires combining the data on network goals with your
knowledge of the physical space. To do so, run through a checklist like
the following:

Get plans for the area the wireless LAN needs to cover. Many
buildings have blueprints available as computer-aided drafting (CAD)
files, but CAD-based processing is overkill in most cases. When you
get the building plans, make sure that either they are to scale, or
the planning tool you are using allows scaling of the drawings. For
drawings that do not have a scale, it is possible to get a rough
scale by labeling a doorway as 3 feet (1 meter) wide, or by taking
the external dimensions of the building.

Divide up the physical space into areas of differing capacity
based on your judgment of expected usage of the network. In
corporate environments, areas where high capacity should drive the
layout include conference rooms, offices, and cubicles. In
educational settings, capacity drivers include classrooms and
lecture halls. In hospitals, areas where wireless LANs support
critical care drive capacity, especially when used with
high-capacity applications like imaging. Be sure to account for the
types of clients in use in each area, and include plans for growth.
As your clients transition from one- and two-stream 802.11n clients
to 802.11ac clients, the demands on the network will grow.

If you’re expecting to support significant usage, be sure to
have usage estimates to match. Do not be afraid of building too
much coverage at this point—it is usually harder to expand a
network than to cut it back.

Estimate your capacity and coverage needs. For each capacity
area in your plan, the estimate requires multiple calculations. When
planning networks, I use several metrics to come up with an AP count
and draw upon my own experience in blending them or choosing between
them. Most importantly, for a high-capacity area, ensure that the 5
GHz band coverage is sufficient. For maximum throughput, neighboring
APs should not be located on the same channel and should be located
as far as possible from adjacent channels.[42] 802.11ac is only supported in the 5 GHz band in large
part because of these advantages. To estimate 5 GHz coverage, you
can use a planning toolset to design coverage with at least a 30 dB
SNR. Or, if you know you know the noise floor within your
environment, you can design the coverage around a signal strength
based on that SNR. In some cases, the manufacturers of devices that
you are targeting for support can supply design criteria. Many voice
device vendors, for example, will suggest that a network be designed
around a signal strength of –67 dBm.

Another estimate of capacity is based on rough
back-of-the-envelope calculations of airtime. As described in “Application Planning”, you can get an extremely
rough estimate of the amount of airtime a device will need by
comparing its total TCP throughput to the application requirements.
With a guess at the number of clients and the airtime consumption of
each, you can derive an estimate of the number of APs
required.

For example, if there are 30 tablets in a classroom and each
tablet requires 4% of the available airtime, then two radios are
required.[43]

To get more precise estimates of your AP capacity
requirements, more accurate test tools are available. Traffic
generators can be programmed with either simulated applications or
application profiles, then installed and run on test devices to
simulate your deployment. In deployments with extensive tablet
usage, be sure to run the traffic generator on a tablet because it
is a single-stream device, often with an antenna system of average
performance. Verifying performance for older physical layers
(802.11a and 802.11n) may also be important for networks that need
to support large numbers of older devices.

For each area that has been estimated separately, add together
each area’s AP count to come up with a total.

As a hard-won piece of practical knowledge, I have found that in
most networks that support general office work and do not have special
demands for high throughput, a standard dual-radio AP can cover about
3,000 square feet. The per-AP coverage area has hovered around 3,000
square feet since the days when 802.11b devices transmitting at 11 Mbps
were considered state of the art. As wireless LAN capacity has
increased, users have moved more applications onto the wireless LAN and
begun to demand much higher quality service.

Initial 802.11ac AP mounting locations

Cabling is one of the biggest costs in placing APs, and the
approach to determining where to put 802.11ac APs will depend on the
extent to which there is existing cabling infrastructure available to
support the network. Reusing an existing cable plant will save a
substantial amount of money because the cost of labor for cabling
installation can be roughly comparable to the cost of the access
points themselves.

If 802.11ac is replacing an existing network based on earlier
technology (802.11a/b/g/n), start by reusing existing cabling and
surveying the area to measure coverage. If the signal quality is
sufficient the existing mounting locations should be acceptable,
although a few additional APs may be required to boost capacity in
“hot spots” where the highest data rates are required. One factor to
watch out for when swapping out older APs for 802.11ac APs is that if
the network is very old and was designed around 2.4 GHz coverage, the
shorter range of 5 GHz coverage may not be sufficient to provide the
desired connectivity.

APs are cheap, and staff time is expensive. Usually, it will
be more cost-effective to replace existing APs in their current
locations and add further capacity if necessary than to take the
time to deliberately re-survey a location for a new 802.11
standard.

If, on the other hand, you are building a wireless network for
the first time, the initial mounting locations should be computed with
some form of planning software. Many product vendors will assist in
the determination of AP locations as part of a project bid process,
often by using wireless LAN planning tools. If you use software to
perform a “virtual” site survey, keep in mind that there is no
substitute for performing either a manual survey with an AP powered on
and measured manually by a target client device, or a rigorous
post-deployment survey to validate the estimates produced by the
planning software. When using software tools, keep in mind that many
basic tools lack the ability to specify user or device density, so be
ready to modify the results of a simulated site survey to adjust them
to your environmental expectations. For example, some tools will
attempt to provide high-quality coverage throughout a designated
coverage area, and it is up to you to move coverage from sparsely used
areas such as hallways and stairwells into the real target usage
areas, such as conference rooms and classrooms.

An upgrade to 802.11ac is also an ideal time to add capacity if
needed. One of the ways in which 802.11ac increases speed is the new
256-QAM modulation, but 256-QAM requires high signal-to-noise ratios.
256-QAM will not work through a wall, so if one of the objectives of
your deployment is to increase the peak throughput available, it may
be necessary to consider putting APs within line of sight of every
place that clients may gather. Planning tools can often estimate the
effects of installing additional APs for capacity purposes, and may
help with setting transmit power levels.

5 GHz coverage and 802.11ac-only APs

802.11ac accentuates the difference in radio range between the
2.4 GHz band and the 5 GHz band. A good rule of thumb is that the
range of a radio is inversely proportional to the square of its
operating frequency.[44] Physical layers at 5 GHz will naturally have a much
shorter range than at 2.4 GHz. In a network designed for 802.11ac
capacity, generally the APs will be placed where they are needed for 5
GHz coverage. In a network designed for 802.11ac capacity, the network
will be quite dense because of the high SNR requirements to support
the 256-QAM rates (MCS 8 and 9). As a result, there are likely to be
places in your network where a dual-radio device does not make sense.
Figure 5-1 illustrates one
example of this. Four APs are used to provide high-quality 802.11ac
coverage. However, due to the longer usable range of 2.4 GHz radio
signals, even when turning the power down, three APs are sufficient to
provide coverage at 2.4 GHz. One of the APs does not need to activate
its 2.4 GHz radio.

Figure 5-1. 2.4 GHz coverage completeness

A common method of adding 802.11ac capacity to an existing
network is to add an 802.11ac radio to a place in space where 5 GHz
coverage needs improvement. Such “infill” APs need only be 5
GHz–capable, but should come from the same vendor as the dual-radio
devices already used on your network to ensure that the roaming, band
steering, and load-balancing capabilities work with the rest of the
network. With 802.11ac having much shorter range, the
capacity-enhancing infill AP is likely going to be an increasingly
large component of your network architecture. If the newly added AP
has dual radios, the 2.4 GHz radio can be used as a full-time sensor.
Applications for sensors are varied, but they include full-time
wireless security sensors and dedicated spectrum monitors. Some
vendors can use such radios as client devices to test actual
performance.

Equipment Selection

With an estimate of the number of APs and their tentative initial
locations, it is time to start picking out an actual implementation,
rather than working with generic APs. At a high level, APs connect the
free-flowing wireless world with the high-performance, fixed-in-place
wired world. After reviewing your network requirements and determining
what constraints drive the logical architecture, it’s time to pick out
your access point hardware. Access points all perform the same basic
function in that they shuttle frames between radio networks and Ethernet
LANs, but there can be tremendous differences in cost and functionality.
Comparing access points on the basis of price alone may prevent you from
discovering a critical feature that improves your ability to manage and
run the network. If you're building a network of more than just a
handful of access points, you probably want to look beyond the hardware
available at electronics stores and at highly functional APs. Here are
some things you may want to consider:

Wi-Fi Alliance interoperability certification

In June 2013, the Wi-Fi Alliance launched an
interoperability program for 802.11ac. Ensuring that your product
vendor has successfully passed interoperability testing is not an
absolute guarantee of interoperability, but it is a strong
statement that the manufacturer believes in interoperability and
has taken steps to ensure compatibility with a wide variety of
client devices. To check on the certification status of a product,
visit the Wi-Fi Alliance
website and click on the “Wi-Fi CERTIFIED Products” button
on the lefthand side of the page.

High performance

Performance is not just a matter of the rate at which
products push data. Many products are capable of pushing "air
rate” data speeds, but only corporate-grade APs have "air rate”
performance while providing a sophisticated feature set under
heavy load. As with many other areas of networking technology,
vendors of corporate-grade hardware invest much more heavily in
software tuning because their products are used in deployments
where more than just the number of bits per second matters. This
investment pays dividends in providing high data rates at longer
ranges from the AP with higher numbers of active client
devices.

Hardware quality and robustness

Corporate-grade devices are designed to be used for many
years before replacement, and therefore are often designed with
future expandability in mind. Components are selected with a view
toward quality and long life, instead of basing decisions
primarily on cost. Sophisticated antennas or other radio frontend
components may be used to improve the quality of the network,
either in terms of throughput or coverage. Radios will be enabled
on all available channels, even though the cost of regulatory
compliance before using DFS channels can be substantial, and
software supports automatic configuration of radio channel
selection. Some deployment areas may require specialized hardware
designs due to either very high or very low operating
temperatures.

Software functionality, upgradability, and quality

Generally speaking, more expensive devices have
significantly more functionality, with advanced features in
several areas. Vendors regularly plan for the release of such
features, and it is common for new features to be provided midway
through a product’s life cycle. Understanding the future
functionality that might be delivered and whether your deployment
would benefit from planned features allows you to consider new
features appropriately in the decision process. Additionally,
extensive QA testing is used to ensure that corporate-grade
devices can be run for months at a time under heavy loads.

Antenna options

Internal antennas allow an AP to be self-contained and to
blend smoothly into the aesthetic environment. External antennas
typically have higher gain, which improves range. In a deployment
based on area of coverage instead of density, or a deployment in a
challenging radio environment, selecting the right external
antenna can make the difference between a poor-quality network and
a successful one. External antennas are also frequently used for
outdoor deployments. Picking the right external antenna is still
something of an art, and the antenna must be matched to the
performance characteristics of the AP. A high-gain antenna will
dramatically increase the transmit range of an AP, but if the AP
has low receive sensitivity, the high-gain antenna will cause more
problems than it solves.[45] Product manufacturers are responsible for obtaining
regulatory authorization for each type of external antenna used,
so a larger selection of external antennas indicates more
extensive regulatory testing.

Power options

Consumer-grade devices are typically powered with a “wall
wart” transformer and must be installed close to existing
electrical outlets, while corporate-grade devices can draw power
from the device at the other end of the Ethernet cable. Power over
Ethernet enables placement of devices in out-of-the way locations,
and can be used to provide power even on very high
ceilings.

Security

Security is not just about providing solid encryption,
though that is the obvious starting point. Corporate-grade
products offer flexible authentication through RADIUS and directory interfaces,
per-user VLAN mapping, traffic filtering and queuing, and built-in
captive web portals for web-based authentication. Fast roaming
support extends the basic encryption to support mobile
applications.

Quality of service

At the most basic level, quality of service support involves
compliance with the Wi-Fi
Multimedia (WMM) certification requirements, which divides traffic
on the air into four classes of differing priority. More complex
queuing systems can be used to improve service quality for voice
devices, or to ensure that airtime is balanced fairly between
network users.

Manageability

If you are reading this book, you need centralized
management. Evaluate management tools for a wireless network in
the same way you evaluate management tools for a wired network.
Ensure that the management software provides something beyond
simple configuration management and can report on the overall
state of the network.

Network Architecture for 802.11ac

Throughout the evolution of wireless LAN technology, there have
been a number of approaches to adding the wireless LAN access layer onto
an existing wired backbone network. Most approaches share two
fundamental attributes, and they remain unchanged by 802.11ac.
Fundamentally, 802.11 provides MAC-layer (or, after the OSI
nomenclature, “layer 2”) mobility. As an 802.11 station moves throughout
the coverage area of the network, from the perspective of the routing
and switching infrastructure it remains in a fixed spot. All
commercially available products that support large-scale networks have
extended the fundamental MAC-layer mobility to encompass the entire
network, sometimes even going so far as to make a single subnet
available in many different locations with VPN technology. Additionally,
ever since the 2006 introduction of WPA2, the 802.1X security framework
(sometimes also called “WPA2-Enterprise” after the Wi-Fi Alliance certification program) has
provided strong authentication and transparent encryption to client
devices. The 802.1X framework offers network administrators the
capability of designing network authentication around user-specific
policies, often assigning a bundle of access rights (variously called a
“profile” or a “role”) to users upon connection to the network.

Many network administrators are familiar with the concept of
protocol layering and the Open Systems Interconnection (OSI) model.
Network protocols are often classified by where they fit in the OSI
model. Less well known, but just as important, is the separation of
network technologies into planes, as shown in the
depth dimension of Figure 5-2. Each plane
has its own protocol layers, of course, but each plane also has a
specialized purpose. Common planes include the following:

Data plane (sometimes called the “forwarding plane”)

Protocols in the data plane move bits from one location to
another and are concerned with moving frames from input interfaces
to output interfaces. In an IP network, the main data plane
protocols are TCP and IP, with applications such as HTTP riding on
top of the network and transport layers.

Management plane

The management plane provides protocols that allow network
administrators to configure and monitor network elements. In an IP
network, SNMP is a protocol in the management plane. A vendor’s
configuration application would also reside in the management
plane; wireless LANs may use CAPWAP as a transport protocol in the
management plane. Without exception, large-scale IP networks use
centralized management and thus have a centralized management
plane. The management plane of the network is responsible for
planning and implementation, policy definition, and ongoing
monitoring.

Control plane

The control plane helps make the network operate smoothly by
changing the behavior of the data plane. An IP network uses
routing protocols for control, while switched networks use the
spanning tree protocol. The control plane of a wireless LAN is
responsible for ensuring mobility between access points,
coordinating radio channel selection, and authenticating users,
among other tasks. The control plane is also responsible for
enforcing policy.

Figure 5-2. Network protocol architecture: layers and planes

Wireless networks can be classified based on the location of the
control plane, and much of the development across the history of
wireless LANs has been about refinements to the control plane. Early
wireless LANs were built out of completely independent APs. The
management plane was practically nonexistent (consisting of the APs’
serial ports and, in a highly engineered network, perhaps a terminal
server), and the control plane was not unified. Networks based on
autonomous APs did not automatically select channels and did not always
support smooth handoff between APs without proprietary protocol
extensions at both ends of the link.

The development of wireless LAN controllers a decade ago led to a
redesign in the way that networks were built, with the control and
management planes being centralized in this new piece of the network. In
a typical controller-based deployment, the access points have limited
functionality without a connection to the controller. Authenticating and
authorizing users is handled by the controller, as are algorithms that
provide RF management functions
such as channel selection. Centralized management and control made much
larger networks possible, and essentially, nearly every large-scale
network built prior to the emergence of 802.11n was built using a
controller-based architecture. In addition to the control and management
planes, early controller-based network architectures centralized the
data plane as well. All data from APs was forwarded through the
controller; this is often referred to as a network
overlay because the wireless network was separate from the
existing core network and existed as a layer on top of the existing
core. In effect, the controller took on the role of a distribution
switch for users attached to APs and provided mobility by serving as an
anchor for the logical point of attachment. Early applications of
wireless LANs were driven by application-specific traffic, not
general-purpose user access, which made the overlay model acceptable to
network administrators.

With the emergence of higher-speed wireless network technologies,
there was a shift in how wireless LANs were used: rather than simply
being small one-off deployments to automate processes, they became
general-purpose access methods. Add-on PC cards were replaced by 802.11
interfaces integrated into the motherboard. With the standardization of
802.11n and 802.11ac traffic volumes have increased dramatically, due to
both the higher speeds and the increase in the number of wireless
devices attached to a typical network. As network load increased,
centralized forwarding through controllers became a traffic bottleneck.
Many vendors responded to the bottleneck by moving the forwarding
decision out of the controller and back to the AP at the edge of the
network, an approach often referred to as distributed
forwarding because the data plane function has moved from
the controller out to the AP, and, in fact, back to a parallel location
with wired traffic. Although this architecture looks superficially
similar to autonomous APs, it is typically paired with centralized
management. Increased processing power also made varying control plane
implementations possible, enabling distributed AP architectures to
handle typical control functions by working among themselves.

Architecture comparison

Building a “micro-network” of an AP or two is easy. With a small
number of APs, it is acceptable to manage the APs individually.
Upgrading to 802.11ac is also straightforward: take out your existing
802.11a/b/g/n APs and replace them with 802.11ac APs. At such a small
scale, almost anything will work. At some point, however, the overhead
of managing individual devices will be too great. At this point, you
are building a small- or medium-sized network. These networks have
just as much to gain from 802.11ac.

Prior to the introduction of distributed APs, most networks
needed a centralized control plane to handle the loads imposed by
large numbers of users, and the choice between autonomous APs and
controller-based APs was a straightforward one that was almost always
resolved in favor of the more advanced centralized control plane. With
the explosion of 802.11 devices now available, network architects have
designed higher and higher capacity networks, stressing the
centralized control plane. Early controller-based networks were able
to use a single controller as the focal point for both the control and
the data plane, but that assumption no longer holds.

Table 5-6 compares the three basic
types of APs described in this section. In reality, there is some
overlap between these architectures when they are implemented in
products. It is likely that a large-scale network at any
speed—especially one supporting critical applications—will require
some degree of decentralization, either by moving some of the data
plane functions to the edge of the network, moving some of the control
plane functions to the edge of the network, or both. All three
architectures are capable of supporting any set of network
requirements, but the cost and availability of the resulting network
may vary.

Table 5-6. Architecture comparison

Attribute

Autonomous APs

Controller-based APs

Distributed APs

Location of data plane

Distributed, enabling high network performance.

Centralized, potentially limiting performance to the
forwarding capacity of a controller. Good mobility support
because devices attach through the controller.

Distributed, enabling high network performance. Many
products have features to assist with mobility.

Location of management plane

Depends on product; often distributed, imposing very
high staff costs.

Distributed, if it exists. Nonexistent control plane
limits flexibility of security and radio management.

Centralized, with high functionality for radio
management and user management.

Distributed. Functionality of control plane depends on
vendor implementation.

Selecting a network architecture

Management plane

If you are building a network consisting of more than a
handful of APs, there is no consideration. Centralized management is
a must, if only because maintaining consistent policy configuration
across multiple devices is easier when you can change network-wide
policies and apply them to devices from a central location, similar
to the way that centralized management tools for wired networks
allow policies to affect the configuration on many devices. Some
early wireless LAN products lacked centralized management, but these
were quickly replaced by products that could be used with a
centralized management system. Many flavors of centralized
management exist, with wide variations in functionality and cost.
Even though centralized management was formerly only accessible to
large-scale networks, the emergence of the software-as-a-service
“rental” model may offer you the ability to use a full-featured
management system at an affordable cost for a small network.

Centralized management is nonnegotiable beyond just a few
access points.

Data plane

The forwarding plane of wireless networks has been the subject
of significant developments over the past five years. When 802.11
first reached the market, it was comparatively slow. Using the
centralized forwarding path in Figure 5-3 did not impose a significant
penalty on the network because wireless LAN speeds were slow enough
for the choke point to keep up. When most 802.11 packets needed
nearly 200 microseconds of preamble to begin transmission, the extra
latency of a trip across the network core was barely noticeable. As
the speed of 802.11 has increased, though, it has become harder and
harder for the centralized forwarding point to keep up.

Figure 5-3. Types of forwarding paths

In practice, there is not a sharp divide between products on
the market that offer a centralized forwarding path and those that
offer a direct forwarding path at the access point. When controllers
are used, the resulting networks may offer the choice of sending
traffic either through the centralized forwarding point or directly
from the AP at the network edge. As speeds have increased, the
ability to offload data forwarding to the edge of the network has
helped keep controllers from becoming bottlenecks on their networks.
At the other end of the spectrum, APs that are generally used in
distributed forwarding deployments typically offer the ability to
make any VLAN accessible throughout the network by using an AP-to-AP
tunnel.

The increased speeds of 802.11ac make AP-level forwarding
much more attractive, especially when combined with the
potential of multi-user MIMO to dramatically increase data
traffic in the future.

Tunnels through the network, whether between an AP and
controller or between APs, must be constructed in a way that is
compatible with existing restrictions on frame size. Client devices
will generally send and receive maximum-length Ethernet frames of
1,500 bytes (though they may of course use 802.11 protocol features
to aggregate several of these frames together). Transporting a
maximum-length Ethernet frame across an intermediate network
requires either that the network support larger frames or that the
tunneling protocol manage fragmentation of the client data frame
plus a tunnel header.

Control plane

In a wireless LAN, the control plane maintains the logical
network attachment of the client, which includes its security
information, the state of any user access rights or service quality
guarantees, as well as path information on how the wireless network
enables data to reach the client. The control plane also manages
coordination between APs for tasks such as radio management and
providing network-wide quality of service. Control plane design is
one of the most fertile grounds for experimentation in wireless LAN
design. The location of the control plane makes an important
contribution to the overall reliability and resiliency of the
network. Building fully redundant wireless networks requires both
resilient data forwarding and resilient control capabilities.

Most large-scale networks were originally built on centralized
control plane technologies, which required that APs be in continuous
contact with a control point. Many centralized control planes are
now moving toward either a split control plane (where functions are
shared between the controller and APs) or a more fully distributed
control plane. Distributed control planes can be cheaper, especially
when designing for distributed networks with many remote sites.
Neither the distributed nor the centralized type of control plane is
inherently more resilient; a distributed control plane protocol can
be resilient by design, while a centralized control plane may
require spare controllers.

Carefully evaluate the trade-offs between a centralized
versus a distributed control plane from the perspectives of
functionality, reliability, and cost.

Hardware Considerations

The Wi-Fi Alliance is
an industry association of companies that collectively drive the
development of wireless LAN technology. The Alliance is best known for
the Wi-Fi CERTIFIED interoperability testing program that began in 2000.
When development begins on new physical layer technologies such as
802.11ac, the Wi-Fi Alliance has a
certification program to ensure that these emerging technologies are
built with interoperability available from the first version. Once
testing is complete and a product is awarded certification, it can be
looked up at the Wi-Fi Alliance
certified product listing. Each product is also given an
interoperability certificate that details the
individual product features that have been certified.[46]

Mandatory tests

Every device submitted for 802.11ac certification must pass a
series of basic tests. The features that are expected to be supported
include:

5 GHz operation

802.11ac is a 5 GHz–only specification. All tests in the
Wi-Fi Alliance certification program require operation at 5 GHz.
This is in contrast to the 802.11n Wi-Fi Alliance certification
program, in which 5 GHz capabilities were optional.

Channel width of 20, 40, and 80 MHz

The initial version of the 802.11ac certification requires
support of all the available channel widths up to 80 MHz. Again,
this is in contrast to the Wi-Fi Alliance’s 802.11n
certification program, which covered only 20 MHz and 40 MHz
channels (with 40 MHz channels being optional).

Dynamic bandwidth signaling

In addition to requiring support of multiple channel
widths, the 802.11ac certification test plan requires
demonstrated interoperability for the dynamic bandwidth
signaling protocol features described in “Dynamic Bandwidth Operation (RTS/CTS)”.

Support of MCS 0 through 7 (up to 64-QAM)

Modulation of up to 64-QAM is required of all devices
seeking 802.11ac certification.

Minimum number of spatial streams

APs must support at least two streams before being allowed
to claim 802.11ac certification; no such rule applies to client
devices. There is an exception for “mobile APs,” which are
battery-powered devices like the Novotel Mi-Fi. Battery-powered
APs are allowed to implement only a single spatial stream. The
number of tested spatial streams is likely to be placed on the
interoperability certificate.

A-MPDU reception

Any Wi-Fi CERTIFIED 802.11ac device must be able to
receive A-MPDU frames. A-MPDU support is typically provided
within the radio chip itself, so support for this option is
widespread. Devices under test are allowed to self-describe the
A-MPDU size supported, so it is impossible to determine the
density of back-to-back MPDUs supported.

A-MSDU reception

In addition to A-MPDU aggregation, to receive
certification devices must support A-MSDU reception.

Security: TKIP & WEP negative tests

802.11ac devices may not use TKIP or WEP to protect frames
sent at 802.11ac data rates. The certification program includes
“negative tests,” which are tests to ensure that WEP and TKIP
cannot be used with 802.11ac data rates. Many products implement
data rate limits when WEP or TKIP is configured, so that if an
802.11ac network is configured for TKIP, its components will
avoid using data rates higher than 54 Mbps.

Optional tests

In addition to the mandatory tests described in the previous
section, the certification program includes a number of optional
capabilities, each of which is called out on the interoperability
certificate:

MCS 8 & 9 (256-QAM support)

When the radio link has sufficient signal quality,
products that implement 256-QAM can achieve throughput of 30%
higher than the mandatory MCS rates.

Short guard interval at 80 MHz

Short guard intervals boost throughput by about 10%, and
their use is widely supported in chipsets. An optional short
guard interval test was defined for use with 802.11n, and the
802.11ac certification extends that test to the wider 80 MHz
channels.

Space-time block coding (STBC)

STBC allows a signal to travel farther because it uses all
of the MIMO signal processing gains to increase range. STBC was
not widely implemented when it debuted with 802.11n, and remains
optional with 802.11ac.

Transmission of A-MPDUs

Support for sending A-MPDUs is optional. This is the only
aggregation test; the certification testing does not validate
A-MSDU behavior.

LDPC

The low-density parity check adds a coding gain of about 2
dB. It is optional within the specification, but a valuable
capability when used with 256-QAM to eke out as much performance
as possible from the radio link.

Like 802.11n before it, 802.11ac comes with a “roadmap” and
several phases to be passed through before full capability is
delivered. Some vendors have delivered modular radios they refer to
as “future-proof” because the radio modules can be upgraded.
Unfortunately, for customers the effect of modular APs is that you
purchase one AP for the price of two and a half APs, and typically
get substandard performance as a bonus for spending the extra
money.

When building a modular AP, designers start with a chassis
that accepts upgraded radios. The chassis defines the system
resources available for the life of the product. (As far as I know,
no modular AP has been produced with an upgradable processor card
like those used in switches and routers.) Designers must build in
extra CPU and memory to provide enough power to accommodate later
upgrades. As a buyer, you pay for more of an AP than you need at the
start to get the extra resources now. Modular APs often cost 50%
more than their fixed-configuration counterparts: you pay for extra
system resources now to preserve the option of upgradeability down
the road.

With luck, product designers have guessed correctly at system
specifications. If the future generation of hardware turns out to be
more capable and resources fall short, performance will be sluggish,
or the vendor will need to eliminate features and deliver a subpar
product. Over the lifetime of a modular AP, the state of the art
will change enough to invalidate design assumptions. An AP chassis
designed before the conception of emerging features will potentially
have the resources to power an 802.11ac upgrade, but it will miss
out on any features that became commonplace after the chassis was
designed. Modular APs suffer from the same problem as other modular
products—the performance is determined by the overall system, and
making just one component better rarely results in the promised
performance benefit.

Another drawback is that when you go to upgrade a modular AP,
there is by definition only one seller. With vendor lock-in, the
cost of the upgrade module may be equivalent to the cost of a new
fixed-configuration AP, designed from the ground up for current
demands. Frequently, purchasers of modular APs find that by the time
they are ready to change
modules, newer fixed-configuration APs cost less but offer greater
functionality.

All this might be worth it if modular APs saved operational
costs, but they do not. Installing modules often requires more work
than changing a fixed-configuration AP because the modular AP needs
to be unmounted, altered, and remounted. In some cases, a new
mounting bracket is needed to ensure the new antennas in the
upgraded module are aligned correctly. The staff cost for adding
modules is usually at least as much, and probably more than, that of
just replacing APs with newer models.

Building an 802.11ac Network

Building a network may begin with detailed information gathering to
make a good prediction of the number and location of APs required, or it
may be more iterative, where a few APs are used to “test the waters” with
a deployment in a key gathering spot for users. In iterative deployments,
using the management capabilities of the wireless LAN system you are
evaluating is a good way to obtain feedback on your assumptions. Is the
client mix what was expected? Are the supposed key applications the most
commonly used applications?

Channel Selection

At first glance, 802.11ac’s addition of yet another channel width
would seem to complicate the configuration process because it means
network designers must manage yet another parameter with backward
compatibility implications. However, the design of 802.11ac’s channel
coexistence mechanisms provides a rough guideline to channel allocation.
Because 802.11ac clients can measure the available bandwidth, an
802.11ac network can take up as much capacity as is available, and two
802.11ac networks sharing the same frequency space can share the wide
channels.

Figure 5-4 shows how a
network can be built with minimum channel overlap. For the purpose of
the figure, each AP’s frequency space is represented by a “stack” of
bars, where the shortest bar is the primary 20 MHz channel, the
next-longest bar is the primary 40 MHz channel, and the longest bar is
the primary 80 MHz channel. When two APs share a channel, the relevant
bar is blended between two colors.

Figure 5-4. Channel addition algorithm for 802.11ac

The figure shows a network being brought up in the following
steps:

When the first AP is powered up, it is straightforward. There
is no existing network, and therefore the AP can choose any channel.
In the figure, the AP represented by blue bars chooses channel 40.
It will therefore take channel 40 for its 20 MHz transmissions,
channels 36 and 40 for its 40 MHz transmissions, and channels 36
through 48 for its 80 MHz transmissions.

The second AP poses no problems, either. There is a free 80
MHz channel from channels 52 through 60, so the AP represented by
green bars chooses, say, channel 60. (All four channels will choose
the non-overlapping 80 MHz channel, so they are all
equivalent.)

When the third AP, represented by orange bars, is added, it
has no free 80 MHz channel. Therefore, it needs to choose a
minimum-interference channel. Stepping down from the desired 80 MHz
channel width, the orange AP can choose the 40 MHz channel of
channels 44 and 48. The overlap between the orange and blue APs is
shown by the way that the 80 MHz channel is blended between orange
and blue.

The addition of the fourth AP, represented by purple, takes a
similar path as the addition of the orange AP in the previous step.
It has no free 80 MHz channel, so it must choose the
least-overlapping 40 MHz channel. The only unoccupied 40 MHz channel
is channels 52 and 56, so it chooses either of those two primary 20
MHz channels as its operating channel. The figure shows it choosing
channel 56.

Finally, when the fifth AP (represented by the color red)
comes up, it cannot choose an unoccupied 80 MHz channel or an
unoccupied 40 MHz channel. Therefore, it must choose a free 20 MHz
channel. In the figure, it is shown occupying channel 48. The 40 MHz
channel composed of channels 40 and 48 is blended between orange and
red to show that it is being shared between those two APs, and the
80 MHz channel is blended between blue, orange, and red to show that
all three APs share the 80 MHz channel.

This process illustrates one important advantage of 802.11ac:
supporting multiple channel widths at the same time enables 802.11ac
clients to “burst” capacity when it's available. Network administrators
should design their networks for minimum channel overlap for wide
channels, and let the narrower transmissions fall where they must to
accomplish that goal. Keeping the wide 80 MHz channels as free as
possible will enable as many fast transmissions as possible from
80–MHz-capable clients and is a worthy goal.

When laying out a network, do not limit yourself to 20 MHz
channels. Lay out the network using the widest channels possible and
spread out the selected channels as much as possible.

Practically speaking, an extensive deployment of 40 or 80 MHz
channels requires support for the worldwide harmonized radio band
(channels 100 to 144 in Figure 2-3). Using
these channels requires that the AP support Dynamic Frequency Selection.
DFS capabilities are required by radio regulators in each individual
country, and support is tested as part of the government certification
process required to sell radio devices.

Network Tuning and Optimization

Part of monitoring the network is watching for conditions that
will lead to substandard service, and, if possible, applying new
configurations to network devices to improve performance and
functionality. Fundamentally, the 802.11 MAC manages airtime. APs turn
available airtime into bits sent to and from the network. Performance
tuning in 802.11ac uses similar techniques to performance tuning in
previous physical layers: reduce airtime contention whenever possible,
and work to pack as many bits as possible into each available
microsecond.

With its emphasis on technologies that assist in improving dense
networks, 802.11ac APs will be packed together quite tightly. Reducing
the coverage area of each AP is an important way of providing more radio
capacity, but it is by no means the end of the story. Even though the
2.4 GHz band is not capable of supporting 802.11ac, it still has an
important role to play as a source of capacity in busy networks. When
serving areas with maximum density, enable
load-balancing features in your wireless network
equipment. Many products support multiple forms of load sharing to
optimize network performance.
Identifying 802.11ac clients, especially those capable of wide channel
operations, and moving them to 802.11ac radios will be an important
component of boosting network capacity. In high-capacity areas, multiple
adjacent APs on nearby channels will need to share capacity.

Many manufacturers select default settings that are generally good
for data networking and will deliver acceptable performance for
web-based applications and email. In fact, many APs include a feature
that gives priority to high-speed 802.11ac frames because they move data
much more quickly than the older 802.11a/b/g/n frames. When transmitting
a 1,500-byte Ethernet frame, 802.11ac is lightning-fast compared to its
predecessors, especially if a wider channel is available for the
transmission. Preferential treatment for fast 802.11ac frames has the
apparent effect of speeding up the network for 802.11ac users with only
minimal impact to users of older devices. The ability of a network to
treat traffic differently to serve the overall user population is often
called “airtime fairness” because when the throughput is optimized for
the entire client population, the result is "fair.”

One important performance tuning technique that is no longer
available to 802.11ac network administrators is control of data rates.
In 802.11a/b/g/n, it was possible for network administrators to control
which data rates were supported. To avoid devices falling back to
airtime-hungry low data rates, network administrators often disable low
data rates. Deactivating low rates often has another second desirable
side effect in that it encourages devices to move off APs with marginal
connections toward better APs. However, the 802.11ac protocol does not
offer control of individual data rates. Devices must support all
non-256-QAM data rates, and the only control offered by the protocol in
the MAC capability information element (see “The VHT Capabilities Information element”) is over the 256-QAM
rates.

The 802.11ac protocol does not provide the capability to control
individual data rates. The only choices available in the protocol are
supporting MCS 0–7, MCS 0–8, or MCS 0–9.

Voice

In contrast to data-oriented networks, some special
configuration may be helpful for networks that support extensive
amounts of voice traffic. Voice traffic is demanding because it cannot
be buffered, so many of the efficiency enhancements in 802.11ac are
not used by voice handsets. The core of voice tuning is reducing
latency for as much traffic as possible. Here are some of the
techniques that can be used:

WMM is a quality-of-service specification that can
dramatically improve the quality of voice at the receiver. Not
all vendors turn on WMM by default, or even make voice the
highest-priority traffic type. The single most important
configuration change you can make to support higher-quality
voice calls is to ensure that WMM is enabled. Some vendors also
have an option for strict priority scheduling, which delivers
frames in order to the receiver.

Enable admission control (WMM-AC)

Admission control requires voice client devices to request
capacity for a call before enabling the call to be established.
For example, a voice handset using G.711 could request that the
AP allocate 80 kbps of capacity. The AP is then free to accept
the request and reserve capacity, or reject the request due to a
lack of capacity.

Enable fast roaming

Multiple techniques for fast roaming may be used, but the
most common are opportunistic key caching (OKC) and 802.11r.
Check with your voice client vendor to figure out which of them
are supported.

Increase data rate used for Beacon frame transmission

Voice handsets are often very aggressive in roaming
between APs, so tuning efforts will focus on decreasing the
effective coverage area of APs and reducing large areas of
coverage overlap. One of the most effective ways of limiting the
effective range of an AP is to make its Beacon transmissions
travel a shorter distance. While it is not possible to design a
radio wave that stops at a certain distance, increasing the data
rate of Beacon frames can be used to limit the effective range
of the network. Typically, the Beacon rate will be set at a
minimum of 24 Mbps, and sometimes even higher. (802.11a/g rates
should be used because many voice handsets do not use
802.11n.)

Shorten DTIM interval

Many voice products use multicast frames for control
features or push-to-talk (PTT) features. Multicast frames are
held for transmission until the DTIM is transmitted.[47] Many APs will ship with a DTIM of 3, so multicast
transmissions are delivered after every third Beacon. Setting
the DTIM to 1 makes multicast delivery more frequent, at the
cost of some battery life on handsets that need to power on
after every Beacon to receive multicasts.

Reduce retry counters

Voice applications are highly sensitive to latency. 802.11
will automatically retry failed transmissions, but
retransmissions take additional time. In voice transmission,
frames should arrive on time or not at all. Using network
capacity to retransmit frames after the target delivery time
does not improve call quality, but it can delay other voice
frames in the transmit queue. Somewhat counterintuitively,
reducing the frame retry count can improve overall latency, and
therefore voice quality.

Multicast

Multicast applications are often similar to voice applications
in terms of the demands placed on the network. Multicast traffic
streams are often video, and may not be easily buffered if they are
real-time streams. Furthermore, multicast traffic has a lower
effective quality of service than unicast traffic on a wireless LAN
because multicast frames are not positively acknowledged. In a stream
of unicast frames, each frame will be acknowledged and retransmitted
if necessary. Multicast transmission has no such reliability mechanism
within 802.11, so a stream of multicast frames may not be received and
there is no protocol-level feedback mechanism to report packet loss.
Here are some steps you can take to optimize multicast
transmissions:

Shorten the DTIM interval

Just as with voice, many multicast applications depend on
receiving data promptly. Setting the DTIM interval as low as
possible improves the latency of multicast delivery.

Increase the data rate for multicast frames

By default, many products will select a low data rate,
often 2 Mbps, for multicast transmissions in an effort to be
backward compatible. While this is a laudable goal, and the
choice of 2 Mbps was reasonable during the 802.11b-to-802.11g
transition in 2004, low data rates for multicast no longer serve
that goal. Unless there are critical applications running on 2
Mbps devices, or there are a large number of such old devices on
the network without any upgrade path, you should increase the
multicast data rate to reduce airtime contention. Many APs can
automatically set the multicast data rate to the minimum data
rate used for unicast frames to associated clients, or even the
minimum unicast rate for clients in the multicast group. With
802.11ac, it is no longer possible to disable the low MCS rates,
so the best that can be done is to disable the low data rates
for previous physical layers.

Enable multicast-to-unicast conversion

Some APs implement a feature that converts a single
multicast frame into a series of unicast frames. Multicast
frames must be transmitted at a rate that can be decoded by all
receivers and therefore is often relatively slow. Unicast frames
can be transmitted much faster if the receivers are close to the
AP. A series of positively acknowledged unicast frames may take
approximately the same amount of airtime, but have significantly
greater reliability.

Internet Group Management Protocol (IGMP) snooping

One of the best ways to limit the load imposed by
multicast traffic is to ensure that it is not forwarded on to
the radio link if no clients are listening. Many APs implement
IGMP snooping, and even if your APs do not, IGMP snooping can be
configured on the switched network connecting the APs. IGMP
snooping monitors membership in multicast groups and only
forwards multicast traffic if there are listeners to the
stream.

Checklist

When planning a network, use the following checklist:

Client count, density, and mix

Gather information on the number of clients you expect to use
the network, and, if possible, what their capabilities are. A good
estimating rule is that an 802.11ac AP can serve around 30–60
clients with acceptable service, depending on the application.
Identify peak data rates that each client will support.

Applications

Identify the key applications that must be supported on the
network. Ensure that these applications are tested during any
proof-of-concept demonstration and before the final acceptance
testing of the new network. Application requirements may also be
used to guide the planning process by working to estimate the number
of APs needed and ensuring appropriate APs to serve high-density
areas.

Backbone switching

Upgrade to gigabit Ethernet at the network edge to connect
your APs, and make sure that the access layer has 10-gigabit uplinks
into the core. Check whether jumbo frame support is required.
10-gigabit Ethernet will not be required for AP connections for the
first wave of 802.11ac, but make sure it is part of your plans as
802.11ac develops. Any new cable runs for 802.11ac should include
two cables.

Power requirements

Supply power to the AP mounting locations. This will need to
be PoE+ (802.3at) for full functionality, so either upgrade edge
switches to use higher power or obtain mid-span injectors to supply
sufficient power to run your chosen AP hardware.

Security planning

802.11ac does not support TKIP or WEP for security. If your
network is not already on CCMP (WPA2), consider moving the network
to use CCMP to avoid needing to reconfigure client devices for the
proof of concept.

After planning the network, as you move into the design and
deployment phases, use the following checklist:

Architecture

The easy choice in architecture is that the management plane
must be centralized. In most cases, a hybrid data plane that blends
aspects of both a distributed data plane and centralized forwarding
will be the right choice. Carefully evaluate the trade-offs for the
location of the management plane based on application requirements
and cost.

Hardware selection

Select hardware that meets your requirements for performance
and functionality and is certified by the Wi-Fi Alliance to ensure
interoperability.

Coverage and capacity planning

Based on the anticipated user density and application mix,
come up with tentative AP mounting locations. Many tools are
available to assist with this process, some of which are free. When
laying out the network, pick the widest “native” channel width for
802.11ac.

[39] Many 802.3af power injectors are able to supply
substantially more power than the specification requires, through
a combination of high-quality components,
shorter-than-maximum-length cable runs, and high-quality cabling.
Even taken together, though, these sources of headroom are only
good for a few watts. The increased resources demanded by 802.11ac
require more than just a few watts, so headroom won’t save you
from a power upgrade.

[40] CCMP is sometimes used interchangeably with the name of the
Wi-Fi Alliance certification program that tests for CCMP
interoperability: Wi-Fi Protected Access, version 2 (WPA2).

[42] With three channels, it is not possible to lay out a
network where neighboring APs do not use adjacent channels. This
constraint is one of the many reasons why the 2.4 GHz band is
not a good choice for a capacity-oriented network.

[43] If 30 devices each require 4% of the available airtime,
you will need 30 x 4% = 120% of the available airtime, or 1.2
radios. Because there is no such thing as a fractional radio,
round up (or, in a spreadsheet, use the “ceiling”
function).

[44] One of the reasons why the TV white space standardization
effort is exciting is that the TV spectrum was around 700 MHz,
giving it a range that can be measured in kilometers instead of
meters.

[45] Receive sensitivity is not commonly reported on data
sheets but may be available in the FCC test reports for
equipment that you are considering.

[46] At the time this book was written, no 802.11ac
interoperability certificates were yet available.